1. Field of Invention
The present invention relates to a laser cavity. More particularly, the present invention relates to a two-mirror 3-dimensional figure-xe2x80x9c8xe2x80x9d ring resonant cavity with the light path resembles a folded rhombus.
2. Description of Related Art
Diode pumped solid-state lasers have wide applications in optical communications, precision measurement, and optical storage. The diode pumped solid-state lasers usually have advantages of compactness and light weight as semiconductor products, and high quality of output mode as solid-state lasers. Since the longitudinal pumping, or also called as end pumping, has better optical conversion efficiency and better mode matching than those of a transverse pumping for low power lasers. The power density within the resonant cavity is greater than that out of the resonant cavity, most of the end pumping solid-state laser uses the intra-cavity frequency-doubling technology to extend the range of wavelength. However, the intra-cavity frequency-doubling technology is encountering a severe, so called, green problem, causing an instability of light output. As a result, it cannot achieve a high purity of laser light such that the applications are greatly limited.
One method to resolve this problem is arranging the laser to be operated at single longitudinal mode. Such method prevents the frequency beating effects so that the laser light output can be stabilized. There are various technologies to produce the single longitudinal mode laser. For example, optical elements can be additionally implemented in the conventional linear cavity or a ring-type resonant cavity can be used to replace the linear-type resonant cavity. If optical elements are implemented inside the linear-type resonant cavity, the single longitudinal output can be only operated at a very low output power level. One unique capability of the uni-directional ring-type resonator as compared to the linear-type resonator, is that the laser light propagates in a form of traveling wave instead of standing wave. Therefore, the spatial hole burning can be eliminated and the green problem can be resolved by using the ring-type resonator.
FIG. 1 is a drawing, schematically illustrating a conventional ring cavity, which is composed of four lenses. In FIG. 1, two pumping beams 102 enter the cavity. Due to the four lenses 90, 92, 94, and 96, the laser light propagates through several typical optical elements known by one skilled in the art, such as the Nd:YVO4 laser medium 104, astigmatic compensator 106, LBO unit 108, bi-refringent filter 110, and Faraday rotator 112. As a result, a green output 114 propagates out from the lens 96.
Another conventional ring cavity is shown in FIG. 2. FIG. 2 is a drawing, schematically illustrating a conventional triangular resonant cavity composed of two lenses and a gain medium. In FIG. 2, an input coupler (lens) 204 and an output coupler (lens) 206 are coupled together. A pump light beam 202 enters the cavity from the input coupler 204 and leaves from the output coupler 206. The light beam 202 is deflected from the output end of the laser medium 210, so that a triangular light path is formed.
FIG. 3 is a drawing, schematically illustrating a conventional ring cavity using a single crystal. In FIG. 3, a monolithic gain medium is cut to have several surfaces, such as five. The laser light generated by the gain medium is reflected by these surfaces, whereby a closed loop light path is formed.
For the conventional ring-type resonant cavities described above, the light path can be formed like co-planar 8-like path. However, the ring-type cavity usually needs more optical elements than a linear resonant cavity. The dimension is larger, and the design is more difficult. For the structure with a single crystal, though the dimension can be reduced, it is still difficult to have the reflection surfaces been precisely cut. Moreover, it still has residual spatial hole burning effect on the input/output surface.
It is an object of this invention to provide a two-mirror 3-dimentional (3-D) figure xe2x80x9c8xe2x80x9d ring-type resonant cavity, which uses two mirrors to form 3-D figure-xe2x80x9c8xe2x80x9d light path, so as to achieve a single longitudinal mode output with small dimension and less optical elements.
The cavity of the invention includes a laser source, a ring cavity, a focusing lens, an optical gain medium, and a nonlinear medium, such as a frequency doubler. The laser cavity is formed by two mirrors preferably with the same curvature. In general, the curvatures of the two spherical mirrors can be different. The curving surfaces of the two mirrors are coupled by a manner of face to face, whereby a space is defined as the ring cavity. The planar convex lens is located between the pumping source and the ring cavity to works as a pair with the input coupler of the ring cavity to focus pumping light. The pumping light enters gain medium where is located at a desired distance of d mm away from the cavity axis. Due to the two curving surfaces of the two mirrors, the light beam is reflected back and forth to form the 3-D figure-xe2x80x9c8xe2x80x9d light path as shown in FIG. 5. The optical gain medium and the nonlinear medium are located on the two arms of this 3-D figure-xe2x80x9c8xe2x80x9d of the light path. The optical gain material is used for generating laser light, and the nonlinear medium is used to do second harmonic generation so as to convert the fundamental laser light to a shorter wavelength.
The pumping light can be generated by a semiconductor diode, but can also be generated by other types of laser. The gain medium can include a three-level type, four-level type, or even an upconversion type. The gain medium can also be glued with the nonlinear medium to form a single unit. According to the desired function, the nonlinear medium can, for example, include a frequency doubler. Alternatively, a saturated absorbing device for a Q-switch laser or a mode-locked laser can also be used. The frequency doubler can also work together with the saturated absorbing device.
The cavity of the invention not only includes the advantages of linear cavity with a small dimension and only few of optical elements, but also includes the advantages of ring-type cavity with stable single longitudinal mode output and flexibility for Q-switching, second harmonic generation or tuning of the single-frequency output.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.